A data communications receiver requires a symbol synchronizer for adjusting the sampling instant at which symbol decisions are made. In a radio data communication receiver, the symbol synchronizer must be resistant to noise and simulcast delay distortion, while, at the same time, it must be capable of tracking out large timing drifts associated with crystal oscillator error over long data frames. In general, there is no definitive "optimum" clock recovery technique, as the best choice of symbol synchronizer is a strong function of the pulse shaping that is used in the end-to-end system and the number of digital signal levels in the case of M-ary signaling.
In simulcast paging systems, for example, pulse shaping is usually kept "loose" in the frequency domain by using, for example, Bessel filtering on both the transmit (pre-modulation filter) and receive (post-detection filter) ends, so that the demodulated eye exhibits a wide eye opening. Using such pulse shaping, it can be shown that, for a 2-level case, the zero-crossings provide ideal timing information, making the traditional zero-crossing PLL used in prior art receivers appropriate for symbol synchronization. The general M-ary case (with M&gt;2), however, exhibits jitter in its zero-crossings, making the zero-crossing PLL used in many prior art 4-level receivers, for example, sub-optimum.
Other prior art techniques have reduced the zero-crossing jitter by allowing timing information to be obtained for only "usable" symbol transitions, i.e., those transitions that produce no zero-crossing jitter. Such techniques can allow excessive timing drift and increased bit error rate when the received data includes too many contiguous non-usable symbol transitions.
Thus, what is needed is a method and apparatus for facilitating symbol timing acquisition in a data communication receiver. Preferably, the method and apparatus will overcome the above described prior art deficiencies for the general M-ary case and will provide a new, jitter-free timing phase error measurement.